Russian Foreign Minister Sergei Lavrov Apr. 2 expressed concern about the U.S. presence in the Black Sea, saying Moscow contacted both Ankara and Washington over the issue amid ongoing tension between Russia and the West.

“We have noticed that recently U.S. military vessels on a number of occasions have extended their presence [in the Black Sea] beyond the established limits and these extensions at times failed to meet the rules of the Montreux Convention,” Lavrov said during a joint press conference with his Kazakh counterpart, Erlan Idrisov, in Moscow.

Lavrov was responding to a question over a U.S. guided-missile destroyer, USS Truxtun, which conducted joint exercises with Romanian and Bulgarian naval forces last month just a few hundred kilometers away from the annexed Crimean peninsula. The Truxtun has already left the Black Sea that had been scheduled before the Ukraine crisis.

“We pointed this out both to the U.S. and of course to Turkey, which is in control of the [Bosphorus] Strait [and the Dardanelles]. Our position is that all the paragraphs of the convention ought to be strictly adhered to. We will monitor it,” Lavrov said. The U.S. Navy is also sending another destroyer into the Black Sea in the coming days, NBC news reported, citing U.S. officials.

The USS Donald Cook, a guided missile destroyer based in Rota, Spain, will travel to the Black Sea “in the next week or so,” according to one senior defense official. The ship will take part in to-be-scheduled exercises with allies in the region, he said.Russia seeks answers over NATO activities

Lavrov also said Russia wanted answers from NATO on its activities in Eastern Europe after the alliance promised to beef up defenses for its eastern members.

Russia’s takeover of Ukraine’s Crimea region last month has caused the deepest crisis in East-West relations since the Cold War, leading the West to impose sanctions and sparking fear President Vladimir Putin has territorial designs beyond the Black Sea peninsula with its Russian-majority population. NATO has ordered military planners to draft measures to reassure nervous Eastern European countries but stopped short of calls by Poland to base more forces there.

Lavrov said any increase in NATO’s permanent presence in Eastern Europe would violate a 1997 treaty on NATO-Russian cooperation. “We have addressed questions to the North Atlantic military alliance. We are not only expecting answers, but answers that will be based fully on respect for the rules we agreed on,” Lavrov said.

Foreign ministers from the alliance met this week to discuss responses to Russia’s Crimea takeover, including sending NATO soldiers and equipment to allies in Eastern Europe, holding more exercises, ensuring NATO’s rapid-reaction force could deploy more quickly, and reviewing NATO’s military plans. NATO military chiefs are concerned that an estimated 40,000 Russian forces near the Ukrainian border may signal plans by Putin to move beyond Crimea into eastern and southern Ukraine, which also have significant Russian-speaking populations.

Lavrov responded to criticism over the size of the force along Russia’s border with Ukraine by saying Moscow had the right to move troops on its territory and they would return to their permanent bases after military exercises. He did not give a timeline for when war games would end but said NATO’s concerns were overblown.

“It is necessary to de-escalate rhetoric which overshoots the mark and crosses into the unreasonable,” he said.